body {
    width: 80%;
    margin: 0 auto;
}

p span {
    display: inline-block;
    background-color: silver;
    width: 100px;
    height: 100px;
    text-align: center;
}
#demo1 {
    width: 100px;
    height: 100px;
    border: 1px red solid;
}